All warm-blooded animals maintain a consistent body temperature. When heat loss from the body is excessive, some source of heat must be supplied for the body temperature to be maintained. When no other source of heat is available, the body tends to shiver in an attempt to provide heat through muscular activity. When the body temperature is excessive, some heat must be removed to maintain proper body temperature.
Typical mechanisms for removal of heat from an inert object are conduction, radiation and convection. These are also operational in the case of living objects. Conduction is apparent when you stand on a cold tile floor in your bare feet. Radiation is typically a poor mechanism in this case since the temperature differentials are relatively small. Convection is apparent when a breeze is present. However, the major mechanism in the case of living objects is evaporation. An amount of energy, known as the latent heat of evaporation, must be supplied when water changes from a liquid state to a vapor state. This is approximately 540 Calories per gram. This mechanism is seen in trees as water evaporates from the surface of the leaves. Since there are many leaves on a large tree, this evaporation is equivalent to several tons of air conditioning for this single large tree and is one reason that a forest feels cooler than an adjacent field.
The primarily mechanism for the cooling of animals is evaporative cooling. The sweat glands in the human body produce sweat, a watery fluid containing sodium chloride and urea, when it is overheated. The eccrine sweat glands are distributed over the entire body but are particularly abundant on the hands, soles of the feet and on the forehead. Apocrine sweat glands are mainly found in the armpits and genital area and also contain fatty material. It is the breakdown of this fatty material that is the primary cause of sweat odor. The vaporization of this moisture removes thermal energy from the body. This is also the primary mechanism for dogs. However, they have few sweat glands and most of the evaporation is from the moist lining of the oral cavity and pharynx. This results in their panting behavior.
The water present on the skin is in equilibrium with the water vapor in the air. Thus, the efficiency of this sweating mechanism depends on the amount of water vapor in the air. Thus we cool off rapidly on dry days but very slowly on humid days. This is also part of the reason why a breeze helps to cool us off—it not only increases the convective cooling but also reduces the higher concentration of moisture around the body. The perceived comfort level in any given atmosphere is related not only to the temperature but also to the efficiency of the evaporative cooling and thus the relative humidity.
The relationship between vapor pressure of water in air versus temperature is shown in FIG. 1 along with a cubic equation that provides an excellent fit over the temperature range shown. This is also includes the normal operating range of most HVAC installations. It is an easy calculation to find the change in relative humidity for a change in temperature when the moisture in the air is held constant. Table 1 shows that for a change of +/−3° C. the relative humidity changes by −/+10% to 15%. Thus, if we were simply cooling the air, the temperature would decrease but the humidity would increase. This would result in a lessened change in perceived comfort. Fortunately in this case, air conditioning systems also remove moisture from the air resulting in a perceived improvement in cooling.
Table 2 gives the moisture in the air as at different temperatures and relative humidifies as a percentage of the moisture at 20° C. and 65% relative humidity. Indeed, if we have a room with constant temperature but a gradient in the relative humidity, our perception is that the room cools off as we walk from the area of high humidity towards the end with lower humidity.
The basic concept for controlling a HVAC system has been to provide a thermostat that turns on the system when an upper set point is exceeded when air conditioning is required. When heat is required, a lower control point is utilized. An improvement is seen in several patents issued for HVAC control systems that are based on a comfort system whereby the control system attempts to adjust the set point temperature based on selected environmental variables including air temperature, humidity, air velocity, clothing insulation, bodily heat production and mean radiant temperature. All of these suffer from the same problems:                They require a large amount of computing power and are slow, iterative calculations poorly adaptive to a control system.        They require feedback from the room occupant as a means of training the system.        They involve complicated variables that actually are relatively constant, at least for a given installation over a long period of time.        The concept of a comfort index is alien to the average homeowner.        They attempt to calculate the dehumidification effects of the HVAC system rather than simply measuring it.What is needed then is a simple to understand and use control system that can be implemented in an inexpensive microcomputer.        